"The case for HS2 is clear. Without it the key rail routes connecting London, the Midlands and northern England will be overwhelmed," he said.

"HS2 will provide the capacity needed in a way that will generate hundreds of thousands of jobs and billions of pounds worth of economic benefits."

Labour shadow transport secretary Maria Eagle called the report a "worrying wake-up call" for the Government.

Stop HS2 campaign manager Joe Rukin said: "The NAO say everything the Stop HS2 campaign has been saying for three years. The Government and MPs haven't wanted to listen to us, but they surely have a duty to listen to the NAO."

Work on phase one of HS2, from London to Birmingham, is set to start in 2016/17 with services beginning in 2026.

Phase two, taking the line north in a Y-shape to Manchester via Crewe and to Leeds via Derby and Sheffield is set to be completed around 2032/33.